Process for preparing cyclopentadienyliron (II) arene complex

Details Process for preparing cyclopentadienyliron (II) arene complex Process for preparing cyclopentadienyliron (II) arene complex

1998-09-18

2001-07-03

Nazario-Gonzalez, Porfirio (Department: 1621)

Organic compounds -- part of the class 532-570 series

Organic compounds

Heavy metal containing

C556S001000, C556S007000, C556S013000, C556S027000, C556S030000, C556S143000

Reexamination Certificate

active

06255507

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to a process of preparing organometallic complexes, specifically cyclopentadienyliron (II) arene complexes.
BACKGROUND
Processes for preparing cyclopentadienyliron (II) arene complexes rely on the ligand exchange reaction of eta-5 dicyclopentadienyliron (II) (commonly referred to as ferrocene) with an arene, in the presence of a Lewis acid. This reaction can be represented as:
where Cp represents a cyclopentadienyl anion, Ar represents an arene or arene anion, and L represents a Lewis acid.
This reaction reportedly involves removal of one cyclopentadienyl anion from ferrocene by the Lewis acid as described and whose structures are given in the
Journal of Organometallic Chemistry Library
1977, 3, 311, and
Tetrahedron
1983, 39, 4037. It is theorized that the reaction produces a coordinately unsaturated cyclopentadienyliron (II) cation and a cyclopentadienyl anion-Lewis acid complex (anion-acid complex). The cyclopentadienyliron (II) cation coordinates with the arene to give the cyclopentadienyliron (II) arene complex product. The cyclopentadienyl anion-Lewis acid complex can undergo further chemistry, the nature of which depends on the particular Lewis acid used. A commonly used Lewis acid for these reactions is aluminum chloride (AlCl
3
). Others include aluminum bromide, gallium chloride, zirconium tetrachloride, hafnium tetrachloride, boron trifluoride and tin tetrachloride. Mixtures of zirconium or hafnium tetrachloride with aluminum chloride and titanium tetrachloride have also been described (See EP-A 314,618 and U.S. Pat. No. 4,868,288).
In one prior art process of preparing cyclopentadienyliron (II) arene complexes, ferrocene is mixed with an arene, a Lewis acid, and a ferrous (Fe
+2
) salt, to produce a cyclopentadienyliron (II) arene cation complex. (See U.S. Pat. No. 5,059,701). The reaction mechanism is said to be as follows: the Lewis acid removes one cyclopentadienyl ligand from a ferrocene molecule resulting in a cyclopentadienyl anion-Lewis acid complex (anion-acid complex) plus a cyclopentadienyliron (II) cation; the cyclopentadienyliron (II) cation complexes with an arene to form the product cyclopentadienyliron (II)arene cation complex; the cyclopentadienyl anion-Lewis acid complex transfers a cyclopentadienyl anion to another ferrous ion to produce another cyclopentadienyliron (II) cation which can undergo complexation with another arene molecule to produce an additional cyclopentadienyliron (II) arene cation complex.
U.S. Pat. No. 5,059,701 specifically and expressly teaches the use of divalent (ferrous) iron (Fe
++
) ion as a reactant, as opposed to iron in any other oxidation state. The ferrous ion is said to react directly with the ferrocene. A described process of providing the ferrous ion is by reducing ferric ion to ferrous ion in a separate, preliminary processing step. The ferrous ion, produced beforehand in a separate step, is then added in the ferrous oxidation state to the other reactants (Lewis acid, ferrocene) to allow reaction of the ingredients toward the cyclopentadienyliron (II) arene complex.
It would be desirable, in the preparation of cyclopentadienyliron (II) arene complex compounds, instead of adding a source of ferrous ion to a reaction mixture, to add a source of ferric ion. This would allow, for one thing, the elimination of the separate, preliminary step described in the U.S. Pat. No. 5,059,701 patent of reducing the ferric ion to ferrous ion prior to reaction with the other reactants. It is described, however, in at least one previous teaching, that ferric ion (as compared to ferrous ion), when in the presence of ferrocene, reacts with the ferrocene to produce ferricenium tetrachloroferrate (III), [Cp
2
Fe]
+
[FeCl
4
]. (See Boeyens et al.,
The reinvestigation of the ferrocene oxidation by iron
(
III
)
chloride in
2-
butanone
-
ethanol
, S. Afr. J. Chem. (1984) 37(1), 32-4). This teaching indicates that ferric ion added directly to reactants including ferrocene would result in production of ferricenium tetrachloroferrate (III), competing with the production of any other desired reaction product, such as a cyclopentadienyliron (II) arene cation complex.
SUMMARY OF THE INVENTION
The invention involves a process for preparing a class of organometallic complex salts, in particular, cyclopentadienyliron (II) arene cation complex salts. According to the invention, ferric ion can be added directly to reactants comprising ferrocene, a Lewis acid, and an arene, to prepare a cyclopentadienyliron (II) arene cation complex. The ferric ion can be converted to ferrous ion in situ, in the presence of ferrocene, with the ferrous ion subsequently reacting with a cyclopentadienylanion-Lewis acid complex to transfer a cyclopentadienyl anion to the ferrous ion and produce a cyclopentadienyliron (II) cation. The cyclopentadienyliron (II) cation can coordinate with an arene to produce a desired cyclopentadienyliron (II) arene complex. Competing reactions between ferrocene and the ferric ion do not lead to an unacceptable level of undesired ferricenium tetrachloroferrate (III) reaction product, and the combination of these reactants produces useful amounts of cyclopentadienyliron (II) arene cation complex.
Briefly, the invention provides a process for preparing cyclopentadienyliron (II) arene cation complexes by combining ferrocene, an arene, a ferric ion, and a Lewis acid. The process adds a source of ferric ion to the reaction solution instead of a source of ferrous ion, as described in earlier teachings; the ferric ion is reduced to a ferrous ion in situ, in the presence of the arene, ferrocene, and Lewis acid, thereby eliminating the separate, preliminary step described in the prior art of forming a ferrous ion from a ferric ion, and then adding the ferrous ion to the other reactants. The ferrous ion thereafter reacts as described in the prior art to produce a cyclopentadienyliron (II) arene cation complex.
A preferred embodiment of reaction sequence can be characterized as the combination of: 1) the in situ reduction of ferric ion to ferrous ion; and 2) a reaction of ferrocene, a ferrous ion, an arene, and a Lewis acid, to produce a desired cyclopentadienyliron (II) arene complex; e.g.,:
 R+Fe
+++
→Fe
++
+R
+
  (1)
2L+(Cp)
2
Fe+FeX
2
+2Ar→2[(Cp)—Fe
+
—(Ar)(LX)
−
]  (2)
In equations 1 and 2 above, R represent a reducing agent, Ar represents an arene, X represents a halogen, and L represents a Lewis acid.
An aspect of the invention relates to a process for preparing a cyclopentadienyliron (II) arene cation complex. The process includes the step of providing a mixture comprising ferrocene, an arene, a Lewis acid, and a ferric ion. The cyclopentadienyliron (II) arene cation complex prepared by the invention can in the form of a salt with counterion A) be represented by the formula:
[(Cp)—Fe—(Ar)]
+
b
A
b−
  (3),
where Cp is an eta
5
complexed cyclopentadienyl anion ligand or an eta
5
complexed indenyl anion ligand, either of which may be substituted or unsubstituted; Ar is an eta
6
complexed arene anion ligand which may be either substituted or unsubstituted; and A is a b-valent anion where b can be 1, 2, or 3.
DETAILED DESCRIPTION
The reaction provides for the production of a cyclopentadienyliron (II) arene cation complex by reacting in a single vessel ingredients comprising ferrocene, an arene, a Lewis acid, and ferric ion; these ingredients will be referred to herein collectively but non-exclusively as “reactants,” and when combined (either prior or subsequent to their reaction), as a “reaction mixture.” Although the reactants can further include other useful ingredients such as an organic solvent, there is no need to directly add a source of ferrous ion.
For purposes of the present description, ferrocene refers to complexes having the general formula:
(Cp)—Fe—(Cp)  (4),
where each Cp independently repres

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